Old to New, New to Old

I have a client that wants a hard drive to work both on his new macbook pro as well as with his old pc laptop (not sure exactly how old). I got a g-tech drive and formatted to MS-DOS, and after copying a few 50MB files over, that seemed to work great. I then tried to copy a 8GB file onto the drive, and I got an error saying the file was too big for the drive format.

Thinking this could have to do with the flash drive I was using, I tried copying the file onto the Macbook Pro, then drag/drop from mac into the HD as it worked with the smaller files, and same error. Never seen this before. Can someone please advise on how to properly set up this HD to work on both laptops and accept all size files? Or if it's not possible please let me know?
Thanks
Eric